A catalytic cracking unit is an important high energy consumption device of an oil refinery at present. A fractionation device cuts products from a catalytic cracking reactor into rich gas, naphtha, diesel oil, slurry and other rough products by using boiling point ranges. An absorption-stabilization system is a post-treatment process of the catalytic cracking unit. It is mainly composed of an absorber, a desorber, a reabsorber, a stabilizer, a corresponding heat exchanger and other auxiliary equipment. The device mainly aims at separating the naphtha and the rich gas produced by the fractionation device into stable gasoline with a qualified vapor pressure, dry gas and liquefied petroleum gas. The detailed separation process is as follows: compressing the rich gas, mixing the rich absorption oil with desorbed gas for gas-liquid balancing on the mixture in a gas-liquid balance tank, introducing the gas at the top of the balance tank into the bottom of the absorber, and introducing the naphtha at the bottom of the tank into the desorber; the top of the absorber discharges lean dry gas, and the gas passes through reabsorber by using light diesel oil as an absorbent to recycle gasoline components; the top of the reabsorber comes out dry gas, and the rich absorption oil at the bottom returns to a main fractionating tower, and the deethanization gasoline at the bottom of the desorber enters the stabilizer. The stabilizer evaporates light components lighter than C4 from the deethanization gasoline, and liquefied gas mainly containing C3 and C4 is obtained at the top; and the bottom product is the stable gasoline with the qualified vapor pressure and is cooled to 40° C., a part of the bottom product returns to the top of the absorber to serve as a supplementary absorbent, and the other part is discharged from the device as products. In order to improve the absorption efficiency of the absorber, the absorber in the absorption-stabilization system is generally provided with an intermediate reboiler for extracting heat at the middle to ensure low temperature absorption, and a large amount of cooled stable gasoline is circulated to the absorber to serve as the supplementary absorbent.
In view of the problems easily occurring to the system that the dry gas carries the liquefied gas and the energy consumption is higher and the like, the objective of the energy saving optimization of the technological process is to reduce the separation energy consumption of the system on the premise of guaranteeing the quality and the yield of the liquefied gas and the stable gasoline. Although a large quantity of studies, such as the patent CN102021033A “Enhanced Mass Transfer And Efficient Energy-Saving Absorption-stabilization System”, and the literature “Development Of Energy Saving Process Of Absorption-stabilization System”, describe such methods to optimize the heat input and circulating water consumption of the absorption-stabilization system as two-stage condensation process, double-strand feeding method, intermediate reboiler process and the like, most of the currently developed methods cannot reduce the absorption temperature of the absorber due to the limitations of the use temperature of circulating water and energy consumption considerations, and thus the reachable energy saving effects are relatively limited. Studies show that the bottleneck of the energy saving method of the system depends on the feeding temperature of the absorber and the intermediate heat extraction temperature, if the feeding temperature and the intermediate heat extraction temperature are reduced, the dosage of the supplementary absorbent can be greatly reduced on the premise of guaranteeing the absorption effect, that is, a large amount of used stable gasoline is circulated in the system.
In view of the above situations, the present invention on the basis of the optimization of catalytic cracking fractionation and absorption-stabilization process parameters, in combination with waste heat refrigeration, residual pressure power generation, waste heat generation and other energy utilization technology, the waste heat of the system itself is used for refrigeration of a driving heat source, the absorption temperature of the absorption process is reduced to lower than 40° C. to reduce the dosage of the circulating absorbent, therefore the heat input, the electrical input and the cooling water consumption of the system can be reduced, and the discharge of a part of waste heat of the system is avoided, which has great significance for the energy saving, emission reduction and industrialization of the catalytic cracking unit.